feat: 实现 iOS Apple Pay 内购支付功能

前端:
- 集成 in_app_purchase 插件，实现 IAP 支付流程
- 添加支付模块 (payments/) 处理产品获取、购买、验证
- 积分中心页面集成 Apple Pay 购买入口
- 设置页面重构: 关于/隐私/协议直接展示，删除 legal_center 子页面
- 修复欢迎引导页滚动检测阈值问题
- 修复解卦结果页 iOS 侧滑返回手势被阻止的问题
- 邀请码绑定按钮临时禁用（待后端实现）

后端:
- 新增 apple_iap_transactions 表记录交易
- 实现 Apple 服务器端验证 (App Store Server API)
- 支付成功后自动发放积分
- 支持 Sandbox/Production 环境切换
- 添加退款处理和交易状态机

协议:
- 更新积分流水协议，支持 purchase/refund 类型
- 新增 PAYMENT_* 错误码

backend/src/v1/payments/router.py

@@ -0,0 +1,45 @@
+from __future__ import annotations
+
+import logging
+from typing import Annotated
+
+from fastapi import APIRouter, Depends, Response
+
+from core.auth.models import CurrentUser
+from v1.payments.dependencies import get_payment_service
+from v1.payments.schemas import (
+    AppleServerNotificationRequest,
+    VerifyTransactionRequest,
+    VerifyTransactionResponse,
+)
+from v1.payments.service import PaymentService
+from v1.users.dependencies import get_current_user
+
+logger = logging.getLogger(__name__)
+
+router = APIRouter(prefix="/payments", tags=["payments"])
+
+
+@router.post(
+    "/apple/transactions/verify",
+    response_model=VerifyTransactionResponse,
+)
+async def verify_apple_transaction(
+    request: VerifyTransactionRequest,
+    service: Annotated[PaymentService, Depends(get_payment_service)],
+    current_user: Annotated[CurrentUser, Depends(get_current_user)],
+) -> VerifyTransactionResponse:
+    return await service.verify_and_grant(
+        user_id=current_user.id,
+        user_email=current_user.email or "",
+        request=request,
+    )
+
+
+@router.post("/apple/notifications", status_code=200)
+async def handle_apple_server_notification(
+    request: AppleServerNotificationRequest,
+    service: Annotated[PaymentService, Depends(get_payment_service)],
+) -> Response:
+    await service.handle_server_notification(signed_payload=request.signed_payload)
+    return Response(status_code=200)